1. Field of the Invention
The present invention relates to apparatus for sampling and analysis of gas phase molecules. A source of negative pressure is used to draw gas phase molecules through a gas-permeable membrane. The gas phase molecules are then directed into an analyzer, suitably a gas chromatograph. The present invention also relates to methods of sampling and analyzing gas phase molecules above and below liquid and soil surfaces
2. Background of the Invention
Surface and subsurface analysis of gas phase molecules in soil and liquid sample sites has traditionally been performed by removing a sample from various levels within a site, and then analyzing it at a later time. In the case of soil or ground water sampling, a special probe is positioned on the lower end of a probe rod string and driven into the ground to the particular level at which a sample is to be taken. Thereafter, the soil sample or ground water sample is removed from the probe and analyzed using various detection and quantifying instruments. As is apparent, if it is desirable to investigate a site to determine the presence and/or quantity of certain compounds or contaminants at a variety of different levels, the use of this direct soil/ground water sampling operation can involve a substantial amount of labor and time. More specifically, to detect compounds or contaminants at various levels the probe rod string must be driven to each particular level, a sample taken, and thereafter the sample removed from the ground by removing the probe rod string. Thus, numerous iterations of driving the probe rod string into the ground and then retracting the probe rod string from the ground are required to sample at the various levels.
U.S. Pat. No. 5,035,149 to Wierenga discloses a soil sampling device called a suction lysimeter. In such a device, a receptacle is implanted in the earth with an air conduit extending from the earth's surface into the receptacle. By drawing a vacuum on the air conduit, soil solution is drawn in from the surrounding soil through the porous walls and collected in the receptacle. A separate conduit for transferring the soil solution sample brings the sample to the surface when positive air pressure is applied to the receptacle through the air conduit. Alternatively, transfer to the surface may be effected by vacuum if from a relatively shallow sampler, although this is said to be less desirable because of the danger of volatilizing components of the sample. The sample is then analyzed for volatile gas phase molecules and other solutes at the surface.
U.S. Pat. No. 5,639,956 to Christy, describes a probe capable of detecting and quantifying chemical compounds and contaminants, particularly volatile compounds which either exist wholly in the gas phase at normal soil temperatures, or exhibit a substantial vapor pressure while existing in the soil in the dissolved, liquid, or solid phases. The '956 patent discloses a permeable membrane soil probe consisting of a gas-permeable membrane having an outer surface in contact with a sample site, and an inner surface in fluid contact with a carrier gas that transports gas phase molecules that have diffused through the membrane to a detector device.
The probe disclosed in the '956 patent, however, is diffusion limited. The amount of volatile gas that diffuses through the gas-permeable membrane and into the carrier gas may be minimal, as positive pressure of the carrier gas relative to the sample site is said to limit the amount of sample gas that can diffuse through the membrane. Therefore, there exists a need for a device that can sample volatile gas phase molecules, at surfaces, as well as below soil and liquid surfaces, without the problems associated with using a high pressure carrier gas.
The present invention fulfills this need by providing a membrane extraction system coupled to a vacuum source to facilitate the sampling of gas phase molecules from a sample and transport them to an analyzer.